The modern high cost of energy has emphasized a cost savings associated with recovery of energy from exhaust fumes in both industrial and domestic settings. For example, modern energy efficient homes use forced ventilation to ensure a healthy change-over of the air in the home. An ideal ventilator will employ a heat exchanger between the vented air and the fresh air so that the ventilation is accomplished with a minimum level of resulting heat loss. Similarly in industrial situations, heat may be removed from waste water and combustion gas exhausts and utilized for space heating, pre-heating of industrial fluids, and pre-heating of combustion gases.
The most efficient heat exchangers are those employing counter-flow heat exchange where two streams of material which are exchanging heat flow in opposite directions through the heat exchanger. In a counter-flow heat exchanger the exhaust stream may reach temperature equilibrium with in-flowing material and the in-flowing material may be warmed to the temperature of the out-flowing material.
Conventional heat exchangers, to function effectively, require that the two fluid streams which are exchanging energy be flowing simultaneously and have approximately the same magnitude and the same heat absorbing capability. This will frequently not be the case, and some form of heat storage is required if the heat in the waste stream is not to be lost. However, conventional heat storage devices do not achieve the efficiency of a counter-flow heat exchanger. As the heat storage medium is warmed, its ability to extract heat from the waste stream is decreased. Phase change heat storage devices have been used to overcome this problem by storing heat at constant temperature. However, phase change materials cannot absorb heat below their characteristic phase change temperature and do not store any portion of the heat at a temperature higher than the characteristic phase change temperature.
My earlier U.S. Pat. No. 4,454,911, which is incorporated herein by reference, discloses a heat exchanger adapted for recovery of heat contained in waste water which is comprised of an outer chamber formed of heat insulating material and an inner bore which is filled with fluids, which acts as a heat transfer liquid medium, and has a high heat capacity. A waste water (warmer fluid) conduit runs through the bore of the chamber in a helical shape, from an inlet end to an outlet end. A supply water (cooler fluid) conduit runs through the bore of the chamber in parallel to but the opposite direction in counter-flow to the waste water conduit. The outer and the inner tubes have a plurality of horizontal and vertical baffle plates mounted within the bore of the chamber in spaced relation to divide it into a plurality of individual compartments, each compartment containing water for heat transfer, and separated from the heat transferring water in adjacent compartments. The baffles perform the further function of providing mounting for the waste water and supply water conduits within the chamber. Within the chamber a temperature gradient is created between each two adjacent compartments within the chamber. The baffle plates are formed of a heat insulating material to minimize conductive transfer of heat between the heat transfer liquid mediums of adjacent compartments.
In a typical home, however, waste heat will be available from such non-continuous flow streams as furnace exhaust gases, dryer exhaust gases, bathroom and kitchen vent air, and waste water. Heat will also be available from continuous fluid streams such as house ventilation air. Intermittent uses for waste heat include domestic hot water, space heating, and the pre-heating of combustion gases. A major use of heat in the home includes the heating of incoming vent air.
Similarly, in industry there will be numerous streams of continuous and discontinuous waste heat and numerous intermittent and continuous requirements for recovered heat. Each home or factory has a different requirement for heat exchange and heat storage, which is dependent on the amount and time-variant nature of the various sources of waste heat and uses thereof.
A counter-flow heat exchanger is desirable which can be used to efficiently recover the heat energy from various gases (such as exhaust air) and fluids (such as industrial waste fluids) and make the recovered energy available to other designated sources at a desired time. What is needed is a heat exchanger with integral heat storage which is capable of non-simultaneous heat transfer between two or more possibly dissimilar fluids.